This invention relates to a material suitable for use as a bone substitute and the manufacturing method thereof, and in particular to a bone substitute useful for repairing body parts subjected to large loads, such as femoral bones, hip joints and teeth roots.
The main criterion considered for an artificial material to be used as a bone substitute is that it exhibits bioactivity, i.e., an ability to bond with bones in the body, more specifically, the material's capability of forming on its surface an apatite layer of the same type as the inorganic components in bone.
Conventionally employed materials for a bone substitute which exhibit such a characteristic are: Na.sub.2 O--CaO--SiO.sub.2 --P.sub.2 O.sub.5 glass, sintered hydroxyapatite (Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2), and MgO--CaO--SiO.sub.2 --P.sub.2 O.sub.5 glass-ceramic. These are superior materials having a bioactivity sufficient to form, in the body, an apatite layer on their surface similar to the inorganic components in bone, and thereby can bond with bones directly. However, the values of fracture toughness, i.e., ultimate strength, of such materials (1 to 2 MPam.sup. 1/2) are not nearly comparative to that of a human cortical bone (2 to 6 MPam.sup. 1/2); therefore, they may not be utilized as a substitute material for reconstruction, replacement or repair of hip joints and tibial bones where a large load is applied.
Therefore, the substitute materials currently utilized for large loads are titanium and its alloys which exhibit the most superior biocompatibility among the metallic materials. These metallic materials exhibit a high fracture toughness; however, an extraordinarily long time, about ten years, is required to directly bond these materials to bones. Thus, to shorten the bonding times, bioactivity has been provided to these titanium materials, by the method of plasma coating, so as to cover their surface with molten hydroxyapatite. The materials obtained by this method offer both the fracture toughness of titanium and the bioactivity of apatite.
However, there are the following problems involved in the use of plasma coating: (1) the necessity of an expensive device for the delivery of the plasma spray, (2) the difficulty of controlling the composition and the crystallinity of the hydroxyapatite formed on the metallic substrate surface since the sprayed hydroxyapatite powder is instantaneously, but only once, exposed to about a 30,000.degree. C. level of high temperature, (3) the difficulty of forming a dense apatite layer since the method deposits the half-molten hydroxyapatite powder on the substrate only by free fall, and (4) the difficulty of forming a strong bond of the apatite layer to the substrate due to the preceding reason.